Cats have vertical pupil slits, while humans have round pupils and goats have horizontal pupils. Have you ever wondered why animals have different eye pupil shapes. The short answer is that it is an adaptation to their environment and way of life. The long answer is a bit more complicated. Here’s a look at why cats have vertical pupils and how other eye shapes confer certain advantages to animals.
- Round pupils provide even focus across the entire depth of field and are most common in daytime-active animals.
- Vertical pupils optimize depth perception and aid animals active in both bright and dim light because of hos tightly they can constrict.
- Horizontal pupils give animals a panoramic field of view that aids in detecting predators during the day.
Why Cats Have Vertical Pupils
Domestic cats have vertical pupils because it gives them advantages suited to their way of life. Vertical pupils give ambush predators, like cats, a superior depth of field so they can judge distance to their prey. The pupil shape helps cats notice side-to-side motions, making rodents and other small creatures easier to spot when they move.
But, not all predators or even all cats have vertical pupils. Lions, tigers, jaguars, and pumas all have round pupils. The big cats typically hunt prey during the day, often from a greater distance than a house cat stalks its prey. Round pupils help big cats because they give the animals clear focus across their entire field of view, but they don’t constrict as tightly as vertical pupils. This leads to another advantage of vertical pupils. They aid small cats in seeing both in bright light and in dim light. So, pupil shape correlates closely to a cat’s hunting pattern. Domestic cats are crepuscular and need to see in both bright light and low light. But, not all small, crepuscular cats have vertical pupils. For example, Pallas’s cat is a wild cat with round pupils.
Another potential advantage of vertical pupils that it acts as camouflage. The slit breaks up the round shape of the eye, making it harder for prey to recognize hidden predators.
Advantages of Different Pupil Shapes
Animals have different pupil shapes that largely correlate to the time of day they are active and their foraging behavior. Humans, dogs, and birds are mainly active during the daytime and look for food and predators that may be either close up or some distance away. Mostly animals that have eyes with vertical slits are crepuscular predators that ambush hunt at dusk and dawn, with some daytime activity. Grazing animals benefit from horizontal pupils because they tend to be active during the day and scan the horizon for threats.
| Pupil shape | Advantages | Active time | Foraging mode | Examples |
|---|---|---|---|---|
| Round | Even focus across entire depth of field | Day | Active foraging, often predator | Humans, birds, dogs, wolves, big cats, some snakes |
| Vertical slit | Excellent depth perception and light intensity control | Day or night | Ambush predator | Domestic cats, foxes, crocodiles, some snakes |
| Horizontal slit | Panoramic field of view | Day or (less often) night | Active foraging, often prey | Goats, cattle, sheep, horses, deer, frogs, toads |
Then, there are animals with other eyes shapes. For example, geckos have pupils with vertical slits that have round openings, resembling beads on a wire. Their eyes identify colors in dim light that animals with round pupils generally can’t see. Cuttlefish have distinctive W-shaped pupils, while rays have crescent-shaped pupils. These shapes give these aquatic species some of the benefits of both round and horizontal pupils.
Pupils Shape Doesn’t Tell the Whole Story
There are other factors besides pupil shape that affect how an animal sees. So, while shape helps make predictions about whether an animal is predator or prey or whether it’s active during the day or night, it doesn’t tell the whole story.
The number and distribution of rods and cones on the retina of the eye affects vision and works in concert with pupil shape. The shape of the lens of the eye also matters. For example, fish have lumpy spherical lenses that help correct chromatic aberration and give them excellent color vision. And, while terrestrial animals with round pupils have mono-focal optics that focus only on the center of the lens, animals with slit pupils have multi-focal optics that sample across the entire diameter of the lends.
References
- Banks, Martin S.; Sprague, William W.; Schmoll, Jürgen; Parnell, Jared A. Q.; Love, Gordon D. (2015). “Why do animal eyes have pupils of different shapes?”. Science Advances. 1(7): e1500391. doi:10.1126/sciadv.1500391
- Brischoux, F., Pizzatto, L. and Shine, R. (2010). “Insights into the adaptive significance of vertical pupil shape in snakes”. Journal of Evolutionary Biology. 23(9): 1878–85. doi:10.1111/j.1420-9101.2010.02046.x
- Land, M.F. (2006). “Visual optics: the shapes of pupils”. Current Biology. 16(5): R167–8. doi:10.1016/j.cub.2006.02.046
- Murphy, C.J.; Howland, H.C. (1990). “The functional significance of crescent-shaped pupils and multiple pupillary apertures”. Journal of Experimental Zoology. 256: 22. doi:10.1002/jez.1402560505
- Roth, Lina S. V.; Lundström, Linda; Kelber, Almut; Kröger, Ronald H. H.; Unsbo, Peter (2009). “The pupils and optical systems of gecko eyes”. Journal of Vision. 9(3): 27.1–11. doi:10.1167/9.3.27